Plug Tracking Using Piezo Electric Pulse Signaling

ABSTRACT

A system for tracking an object in oil and gas wellbore operations wherein a releasable object carrying a first signal system is released into tube system associated with a wellbore. The first signal system communicates with one or more second signal systems positioned along the travel path of the object; along the surface of the formation; and/or throughout the wellbore. First signal system and the second signal system may communicate by RF signals. First signal system and any second signal systems positioned on the surface communicate by through-the-earth or very low frequency signals. A global positioning system may be utilized in conjunction with any second signal systems on the surface to identify the absolute location of the object in the underground wellbore. The first signal system carried by the object may be a piezoelectric system disposed to transmit a signal when the object experiences a predetermined pressure.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to methods of tracking the release and movementof one or more plugs, balls, darts or similar devices in a pipe or tubesystem of an oil or gas well. Such systems may include drill, completionor production strings disposed in wellbores, whether cased or uncased,whether single or multi-lateral in nature (any such system is referredto in this specification and the claims simply as “tube system”).

Description of the Prior Art

It is well known in the art to use plugs, balls, darts or similardevices released into an oil and gas tube system to accomplish one ormore tasks (any such plugs, balls, darts or similar device is referredto in this specification and the claims simply as a “ball”). Such tasksmay include separating a displacement fluid from another fluid in adownhole operation or downhole tool actuation.

A fluid of a particular type, composition, viscosity and/or otherphysical properties is frequently displaced through a pipe by a secondfluid of a different type, composition, viscosity or other property.Very often, it is necessary to displace a first fluid through a pipe bya second fluid without mixing the two fluids. This has heretofore beenaccomplished by physically inserting a resilient plug or ball betweenthe two fluids. The plug functions to separate the fluids, preventingthem from being mixed and also to wipe the walls of the pipe and removeresidue therefrom as the first fluid is displaced through the pipe bythe second fluid. For example, in cementing operations, a displacementfluid is used to push cement slurry through the tube system.Specifically, well pipe used to case wellbores is cemented into thewellbore to anchor the well pipe and isolate differently pressured zonespenetrated by the wellbore. Pipe used for this purpose is generallyreferred to as “casing.” The cementing step is initiated by pumping acement slurry down into the casing from the well surface. The cementslurry flows out from the bottom of the casing and returns upwardlytoward the surface in the annulus formed between the casing and thesurrounding wellbore.

In the cementing process, the fluid normally used in the drilling of thewellbore, referred to herein generally as “drilling fluid,” is displacedfrom the casing ahead of the cement slurry pumped into the casing. Whena sufficient volume of the cement slurry has been pumped into the wellpipe, drilling fluid is used to displace the cement from the well pipeto prevent the pipe from being obstructed by the cured cement.

The drilling fluid and cement slurry are separated during thedisplacements with appropriate liquid spacers, or more preferably, withresilient, sliding wiper plugs or balls that seal along the inside ofthe well pipe and isolate the cement slurry from the drilling fluid.When using wiper plugs to separate the drilling fluid and cement slurry,the cement slurry is pumped behind a first wiper plug to push the plugthrough the casing, forcing the drilling fluid in the casing to flowahead of the plug. As the plug moves along the pipe, it wipes the innersurface of the pipe to remove debris that could mix with the slurry. Thedrilling fluid displaced from the bottom of the casing flows upwardlythrough the annulus and returns toward the well surface.

When a sufficient volume of cement has been pumped behind the firstwiper plug, a second wiper plug is positioned in the casing and drillingfluid is pumped into the casing behind the second plug to push thecement slurry through the casing. A flow passage in the first plug openswhen it reaches the casing bottom to permit the cement slurry to flowthrough and past the plug, out the casing bottom. Once the first wiperseal has been opened and its seal terminated, the continued advance ofthe second plug through the casing displaces the cement slurry past thefirst plug, around the end of the casing, and up into the annulus. Thesecond plug stops and maintains its sealing engagement with the casingonce it arrives at the bottom of the casing.

In other operations, a downhole tool or mechanism may be designed to beactuated by the application of a predetermined fluid pressure applied tothe tool. In order to accomplish this, a plug, ball, dart or similardevice is pumped down the tube system and used to temporarily increasethe fluid pressure within the tube system at a desired location with theincrease pressure utilized to actuate a downhole tool or mechanism. Forexample, during the stimulation of subterranean wells, a productionsliding sleeve having ports is introduced into the well bore forfracturing, acidizing, or other treatment applications. A number ofsleeves may be run on a single production string. The sleeve(s) may beoperated by either a mechanical or hydraulic shifting tool run on coiledtubing or on jointed tubing using a ball-drop system. In the ball-dropsystem, a ball is dropped into the well bore and then fluid pumped intoa portion of the sleeve at a sufficient pressure such that the balllands on a baffle or seat, causing a pressure increase in the fluid. Asthe fluid pressure increases, the pressure causes the sleeve to open.Once the sleeve is opened, the ports of the sleeve align with ports inthe production string and fluid flow is diverted through the ports.

In any of the ball drop operations described above, it is important toknow where the ball is in the tube system. In the case of cementingoperations, the balls are generally placed into the well at the surfaceusing ball injector apparatus or released from a plug container.Ensuring the positive release of the cementing plug from the plugcontainer is critical to the cementing operation since the release isused by the operator to measure the volume of cement being pumpeddownhole.

Typical prior art cementing plug containers utilize a mechanical leveractuated type plug release indicator linked to an external flipper toindicate the passage of the cementing plug from the cementing plugcontainers. In some instances, these prior art mechanical lever actuatedtype plug release indicators may indicate the passage of the cementingplug from the cementing plug container, although the cementing plug isstill contained within the container. The failure to properly releasethe cementing plug from the cementing plug container can ruin anotherwise profitable well cementing job due to the over-displacement ofthe cement to insure an adequate amount of cement has been pumped intothe annulus between the casing and wellbore. Likewise, the mechanicallever or flipper paddle on the inner diameter of the plug container canoften damage the plug as it passes through. In addition, smaller ballsor objects will not always activate the flipper.

Another type of cementing plug indicator utilizes a radioactive nailplaced into the cementing plug in the cementing plug container. When thecementing plug having the radioactive nail lodged therein is no longerpresent in the cementing plug container, a radiation measuringinstrument, such as a Geiger counter, will not react to the radiationemitted from the radioactive nail in the cementing plug therebyindicating that the plug is no longer in the cementing plug container.However, since the shelf life of readily available and easily handledradioactive nails is limited, such nails may be difficult to obtain andstore, when working in remote areas.

Additionally, an acoustic type plug release indicator can be utilized inwhich a microphone detects the sound of the plug moving through the wellcasing and transmits the signal to an operator listening system and amagnetic tape recorder.

Once a ball is released into a tube system, existing downhole objectssuch as plugs, balls and darts have no way of communicating theirlocation to the surface except through pressure spikes that result whenthe object encounters a restriction. To the extent the object passesthrough the restriction, pressure may spike and then diminish once theobject has passed through the restriction. To the extent the objectbecomes lodged in a restriction, pressure will spike and remainelevated. Likewise, to the extent the object lands on the desired seat,pressure will spike and remain elevated. However, there is no way forthe surface operator to know if any particular pressure spike is from anobject that has landed as desired or from an object that may have becomelodged or stuck in the tube system above the desired seat. In otherwords, to assess the movement of an object through the tube system, anoperator can attempt to interpret changes in pressure. Moreover, whilesuch a method may indicate when an object has landed on a seat, themethod provides very little feedback with respect to the movement of theobject through the tube system. Thus, there is a need for a system andmethod for more accurately tracking the release and movement of a ball,plug, dart or similar object moving through a wellbore system.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure will be understood morefully from the detailed description given below and from theaccompanying drawings of various embodiments of the disclosure. In thedrawings, like reference numbers may indicate identical or functionallysimilar elements.

FIG. 1 is a plan view of a marine based production system having areleasable object tracking system of the disclosure.

FIG. 2 is a plan view of a cement head assembly incorporating areleasable object tracking system of the disclosure.

FIG. 3 is a plan view of a releasable object and signal transmissionsystem of the disclosure.

FIG. 4 is a plan view of a land based drilling system having a very lowfrequency system for tracking a releasable object in a wellbore.

FIG. 5 is a plan view of a marine based drilling system having a verylow frequency system for tracking a releasable object in a wellbore.

FIG. 6 is a plan view of a land based drilling system having a GPSsystem for tracking a releasable object in a wellbore.

FIG. 7 is a flowchart of a method of utilizing a very low frequencysignal to track a releasable object in a wellbore.

FIG. 8 is a plan view of a releasable object and piezoelectric signalsystem of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The disclosure may repeat reference numerals and/or letters in thevarious examples or Figures. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Further, spatially relative terms, such as beneath, below, lower, above,upper, uphole, downhole, upstream, downstream, and the like, may be usedherein for ease of description to describe one element or feature'srelationship to another element(s) or feature(s) as illustrated, theupward direction being toward the top of the corresponding figure andthe downward direction being toward the bottom of the correspondingfigure, the uphole direction being toward the surface of the wellbore,the downhole direction being toward the toe of the wellbore. Unlessotherwise stated, the spatially relative terms are intended to encompassdifferent orientations of the apparatus in use or operation in additionto the orientation depicted in the Figures. For example, if an apparatusin the Figures is turned over, elements described as being “below” or“beneath” other elements or features would then be oriented “above” theother elements or features. Thus, the exemplary term “below” canencompass both an orientation of above and below. The apparatus may beotherwise oriented (rotated 90 degrees or at other orientations) and thespatially relative descriptors used herein may likewise be interpretedaccordingly.

Moreover even though a Figure may depict a horizontal wellbore or avertical wellbore, unless indicated otherwise, it should be understoodby those skilled in the art that the apparatus according to the presentdisclosure is equally well suited for use in wellbores having otherorientations including vertical wellbores, slanted wellbores,multilateral wellbores or the like. Likewise, unless otherwise noted,even though a Figure may depict an offshore operation, it should beunderstood by those skilled in the art that the apparatus according tothe present disclosure is equally well suited for use in onshoreoperations and vice-versa. Further, unless otherwise noted, even thougha Figure may depict a cased hole, it should be understood by thoseskilled in the art that the apparatus according to the presentdisclosure is equally well suited for use in open hole operations.

Generally, in one or more embodiments, a method and system for trackingobjects released into a tube system of a wellbore is provided whereinthe object carries a first signal system, which may be either a signaltransmitter, a signal receiver, or both. Deployed at one or morelocations throughout or in proximity to the wellbore tube system is oneor more second signal systems, which may be either signal receivers,signal transmitters or both, in order to communicate with the firstsignal system. In one or more embodiments, one signal system is an RFIDchip and the other signal system is an RFID reader. In one or moreembodiments, the releasable object carries a signal transmittercomprising an RFID chip. In this case, the second signal systemcomprises an RFID reader which may be positioned along the tube system,such as adjacent an object release, and identifies the object as itpasses the RFID reader Likewise, the one or more second signal systems,such as RFID readers, may be positioned along the tube system of thewellbore and disposed to identify when the releasable object passes aparticular location. In one or more embodiments, the signaltransmitter(s) may each be a magnet and the signal receiver(s) may eachbe an electromagnetic sensor. For example, a magnet may be attached toor carried by the releasable object and electromagnetic sensors may bepositioned along the wellbore. In one or more embodiments, one signalsystem is simply an electromagnetic (EM) transmitter that communicateswith a network of at least two and preferably three or more of a secondsignal system comprised of electromagnetic receivers positioned adjacentthe surface and each having global positioning system (GPS) verifiedlocations. As the releasable object moves through the wellbore, thesurface receivers receive an EM signal and determine the location of theball in the wellbore. In one or more embodiments, the signal transmitteris an EM transmitter emitting EM signals in the very-low frequency (VLF)range (approximately 3-30 kilohertz (kHz). A VLF receiver is positionedat the surface as a second signal system and the signal transmittertransmits a VLF signal to the surface at predetermined time intervals.In one or more embodiments, the first signal system carried by thereleasable object includes a piezoelectric system which emits a signalbased on pressure applied to a piezoelectric element of thepiezoelectric system.

Turning to FIG. 1, shown is an elevation view in partial cross-sectionof a wellbore drilling and production system 10 utilized to producehydrocarbons from wellbore 12 extending through various earth strata inan oil and gas formation 14 located below the earth's surface 16.Wellbore 12 may be formed of a single or multiple bores 12 a, 12 b . ..12 n, extending into the formation 14. Wellbore 12 may include one ormore casing strings 18 cemented therein, such as the surface,intermediate and production casing shown in FIG. 1.

Drilling and production system 10 includes a drilling rig 20. Drillingrig 20 may include a hoisting apparatus 22, a travel block 24, and aswivel 26 for raising and lowering casing, drill pipe, production tubingor other types of pipe or tubing strings 30.

Drilling rig 20 may be located proximate to or spaced apart from a wellhead 32, such as in the case of an offshore arrangement as shown. One ormore pressure control devices 34, such as blowout preventers and otherequipment associated with drilling or producing a wellbore may also beprovided at wellhead 32.

For offshore operations, whether drilling or production, drilling rig 20may be mounted on an oil or gas platform 35, such as illustrated in theoffshore platform shown in FIG. 1. Although system 10 is illustrated asbeing a marine-based system, system 10 may be deployed on land. In anyevent, for marine-based systems, a subsea conduit 36 extends from deck38 of platform 34 to a subsea wellhead 32. Tubing string 30 extends downfrom drilling rig 20, through subsea conduit 36 and into wellbore 12.

A working or service fluid source 40 may supply a working fluid pumpedto the upper end of tubing string 30 and flow through tubing string 30.Working fluid source 40 may supply any fluid utilized in wellboreoperations, including without limitation, drilling fluid, cementiousslurry, acidizing fluid, liquid water, steam or some other type offluid.

Wellbore 12 may include subsurface equipment 42 disposed therein, suchas, for example, a completion assembly or some other type of wellboretool.

Wellbore drilling and production system 10 may generally becharacterized as having a pipe system 50. For purposes of thisdisclosure, pipe system 50 may include casing, risers, tubing, drillstrings, completion or production strings, subs, heads or any otherpipes, tubes or equipment that attaches to the foregoing, such as string30 and conduit 36, as well as the wellbore and laterals in which thepipes, casing and strings may be deployed.

In one or more embodiments, an object release 52 may be deployed alongthe pipe system 50 for release of a releasable object 54 into the pipesystem 50. For purposes of the disclosure, the term “releasable object”or “object” is used to refer to plugs, balls, darts or similar objectsthat may be released into a tubing string or wellbore. The object 54 isgenerally characterized as formed of a body with no surface 16 ordrilling rig 20 attached guiding mechanisms (such as a wireline ortubing) for guiding or urging the body down wellbore 12. Except forspecific embodiments which are described below, the body of object 54 isnot limited to any particular shape. While object release 52 may bedeployed adjacent drilling rig 20, in other embodiments, object release52 may be deployed at any other location of drilling and productionsystem 10, such as along a riser or conduit 36 or at a wellhead 32 orblowout preventer 34.

Object 54 carries a first signal system 44 disposed to communicate withone or more second signal systems 46 deployed in association withdrilling and production system 10 as described in more detail below. Inthis regard, at a location above the wellhead 32, one or more secondsignal systems 46 may be deployed along pipe system 50 asabove-the-wellhead second signal system 48. Alternatively, or inaddition thereto, one or more second signal systems 46 may be deployedin wellbore 12 along pipe system 50 as wellbore second signal system 58.Alternatively, or in addition thereto, one or more second signal systems46 may be deployed at or in proximity to surface 16 as surface secondsignal systems 60. Second signal systems 48, 58 and/or 60 may be signalreceivers or signal readers in some embodiments. Alternatively, secondsignal systems 48, 58 and/or 60 may be signal transmitters in someembodiments.

In any event, one or more ball seats or landing collars 56 may bedeployed along the pipe system 50 for receipt of an object 54 during aparticular wellbore operation.

FIG. 1 also illustrates surface mounted equipment 62 of a drilling orproduction system 10. Persons of ordinary skill in the art willappreciate that the disclosure is not limited to a particular type ofsurface mounted equipment, but generally refers to any type of equipmentmounted above the wellhead 32. Such surface mounted equipment may be anobject release 52.

One embodiment of surface mounted equipment 62 is illustrated in FIG. 2as a cement head assembly 64 that incorporates an object release 52, butit is understood that cement head assembly 64 is provided forillustrative purposes of one embodiment only.

Thus, cement head assembly 64 generally includes a cement head sub 66and, optionally, an upper safety valve system 68 and a lower safetyvalve system 70. Cement head sub 66 is an elongated tubular 78 having aninner bore 72 extending therethrough. Cement head sub 66 includes alower or first object chamber 74 and an upper or second object chamber76, each disposed for receipt of an object 54 for release into pipesystem 50. One or both of chambers 74 and 76 may be comprised of aportion of inner bore 72 or may be separately formed and incommunication with inner bore 72. An object release mechanism 80 isdisposed in proximity to each of first chamber 74 and second chamber 76to secures objects 54 a, 54 b in their respective chambers and which canbe activated to release object 54 through inner bore 72.

In some embodiments, chambers 74 and 76 each comprise a portion of innerbore 72.

Associated with lower object chamber 76 is a lower release mechanism 80aand associated with upper object chamber 74 is an upper releasemechanism 80b. Each release mechanism 80 includes a release element 82movable between a first position (closed) to secure an object 54 in anassociated chamber 74, 76 and a second position (open) to release anobject 54 from the associated chamber 74, 76. In some embodiments,movable release element 82 is a rotatable cylindrical element having afirst radial through bore 84. In some embodiments, rotatable cylindricalelement 82 may also include an internal flow passage 86. Rotatableelement 82 is radially positioned at the lower end of each chamber 74,76 and rotatable between a first closed position (shown) in which bore84 is offset from bore 72, and a second release position (not shown) inwhich bore 84 is aligned with bore 72.

Release mechanism 80 preferably also includes a position indicator, suchas is illustrated by lower indicator 88 and upper indicator 90. Eachindicator 88, 90 provides an external visual indication of the alignmentof bore 84 relative to bore 72.

Persons of ordinary skill in the art will appreciate that while objectrelease 52 is illustrated with two chambers 74, 76, object release 52may include fewer chambers or more chambers. For example, a thirdchamber with a corresponding release mechanism may be included.

Positioned along elongated tubular 78 below lower chamber 76 is aflipper mechanism 92. Flipper mechanism 92 generally includes an arm orextension 94 movably mounted on tubular 78 so that arm 94 protrudes intobore 72 when arm 94 is in a first position and is at least partiallyretracted from bore 72 when arm 94 is in a second position. Linked toarm 94 is a visual indicator 96 mounted on the exterior of tubular 78.Visual indicator 96 is movable between a first position corresponding tothe first position of arm 94 and a second position corresponding to thesecond position of arm 94. As an object 54 moves past arm 94 in bore 72,the object 54 drives arm 94 from its first position to its secondposition. Visual indicator 96 thereby provides an indication that anobject 54 has moved past arm 94 following release of object 54 from itscorresponding chamber.

Positioned along elongated tubular 78 below lower chamber 76 are one ormore second signal systems 46 disposed to communicate with a firstsignal system 44 carried by object 54. In one or more embodiments, oneof the signal systems 46, 44 is a signal transmitter disposed totransmit or emit a signal that can be used to identify the object 54,while in other embodiments, one of the signal systems 46, 44 is a signalreceiver disposed to receive a signal that can be used to identify theobject 54.

In one or more embodiments, the above-the-wellhead second signal systems48 is positioned below flipper mechanism 92 to ensure object 54 passesflipper mechanism 92 upon release. In any event, in one or moreembodiments, first signal system 44 carried by the object 54 may be asignal transmitter in the form of an RFID chip and second signal system46, such as above-the-wellhead second signal systems 48, may be a signalreceiver in the form of an RFID reader which may be positioned toidentify object 54 as it passes the RFID reader. In one or moreembodiments, the first signal system 44 may be a signal transmitter inthe form of a magnet attached to or carried within the object 54, andabove-the-wellhead second signal systems 48 positioned along elongatedtubular 78 may be a signal receiver in the form of an electromagneticsensor. In one or more embodiments, the first signal system 44 is simplyany device or material that emits measureable magnetic field orelectromagnetic (EM) energy as a “signal” and the second signal system46 is any device, such as a sensor, disposed to measure or otherwiseidentify the EM energy or “signal” emitted by first signal system 44. Inthis regard, object 54 may be formed of a material that emits a magneticfield or EM energy. In one or more embodiments, the first signal system44 may transmit discreet signals unique to the particular object 54 withwhich the first signal system 44 is associated. Thus, in the case wheremultiple objects 54 might be used in an operation, the first signalsystem 44 of each object may transmit a signal different or separatelyidentifiable from the signals of the first signal systems 44 of theother objects 54. For example, in the case of an operation utilizing twoobjects 54 a, 54 b, the first signal system 44 of the first object 54 awill transmit a first signal and the first signal system 44 of thesecond object 54 b will transmit a second signal different from thefirst signal. In one or more embodiments, the second signal system 46 issimply any device that emits measureable magnetic field orelectromagnetic (EM) energy and the first signal system 44 is anydevice, such as a sensor, disposed to measure the EM energy emitted bythe second signal system 46. As used herein, “signal system” may be asignal transmitter, a signal receiver, or both a signal transmitter anda signal receiver. Thus, the configuration of signal transmitters andsignal receivers may be reversed, with signal receivers carried by anobject and signal transmitters positioned along the wellbore. In anyevent, the disclosure is not limited by the manner in which signals aredistinguished. Thus, signals may be different in frequency, phase oramplitude, among other methods for differentiating signals generallyknown in the art.

In one or more embodiments, second signal system(s) 46 may be hard wiredto a monitoring system 102 remote from or positioned at a locationremoved from the proximity of the surface mounted equipment 62, while inother embodiments, one or more second signal systems 46 may include awireless transmitter 104 forming a wireless network disposed towirelessly communicate with monitoring system 102 or with other secondsignal systems 46. Without limiting the foregoing, monitoring system 102may be a computer system, a control system, a handheld or portabledevice such as a tablet or smartphone, or some other type of monitoringor control equipment.

Although object 54 is not limited to a particular type or configuration,FIG. 3 illustrates one embodiment of object 54. Object 54 of FIG. 3illustrates first signal system 44 as a signal transmitter. As shown inFIG. 3, in one or more embodiments object 54 may include an elongatedtubular body 106 having a first end 108 and a second end 110 with a bore112 formed therein. In one or more embodiments, bore 112 may be athroughbore or passage or channel to allow fluid to pass through or byobject 54. Disposed along the outer surface of object 54 are one or morewipers 114 having an outwardly extending flexible lip 116. Lip 116 maybe formed of a pliable or resilient material, such a rubber. An end cap118 is mounted on first end 108 and may include an aperture 120 incommunication with bore 112. First signal system 44, or a portionthereof, may be mounted along or within bore 112. In one or moreembodiments, first signal system 44 may include a throughbore 128 influid communication with bore 112 so that fluid entering object 54through aperture 120 may pass through object 54. In any event, firstsignal system 44 may be secured to object 54 by a fastener 124,preferably adjacent the second end 110 of object 54 so that a secondsignal system 46 disposed in pipe system 50 (see FIG. 1) preferably willnot will not receive a signal from the object 54 until the object 54 hassubstantially moved past the second signal system 46. In one or moreembodiments, first signal system 44 may be a cylindrical shaft 126formed of a magnetic material or EM emitting material. In the case werefirst signal system 44 is a cylindrical shaft 126, cylindrical shaft 126may include throughbore 128. In one or more embodiments, fastener 124may be an externally threaded ring disposed to engage internal threadsdisposed in bore 112 adjacent second end 110 of object 54. In one ormore embodiments, the signal from first signal system 44 may beunidirectional or otherwise shielded, such as by shielding 130, so thatthe receiver 46 only receives a signal after a desired portion of object54 has moved past second signal system 46. For example, a unidirectionalsignal transmitted behind or upstream of object 54 would only bedetected by second signal system 46 once object 54 has moved past thelocation of second signal system 46. In another example, shielding 130may be disposed to limit the direction of propagation of a signalemitted from first signal system 44. In one or more embodiments, controlelectronics 132 and a power source 134 may also be included as part offirst signal system 44.

In operation, first signal system 44 carried by object 54 is activated,as necessary, prior to release from or through the surface mountedequipment 62. It will be understood that in some cases, such as wherefirst signal system 44 is magnetic material carried by object 54 orotherwise forming object 54, no activation is necessary. In any event,once first signal system 44 is activated or otherwise emitting a signal,object 54 is released into pipe system 50. In one or more embodiments,object 54 may be released from an object release 52. In the illustratedembodiment, object 54 is released into the elongated tubular 78 ofcement head sub 66. When object 54 passes the second signal system 46, asignal emitted from object 54 is received by second signal system 46,triggering a signal that is transmitted, either via a wired or wirelesstransmission system, to monitoring system 102, permitting verificationthat the object 54 has passed the location of the second signal system46. To the extent second signal system 46 is above the wellhead 32 inthe form of above-the-wellhead second signal system 48, communicationsbetween above-the-wellhead second signal system 48 and first signalsystem 44 occur by wired or wireless signal transmission.Above-the-wellhead second signal system 48 may be positioned adjacentobject release 52 or along subsea conduit 36 or adjacent wellhead 32. Inthis way, operators can know with certainty that object(s) 54 has beenreleased from or has otherwise passed through surface mounted equipment62, such as plug containers, cementing subs, top drive heads or thelike, thus allowing a particular procedure to continue or ensuring theobject 54 has passed a particular above-the-wellhead location.

For example, in a cementing operation, a first plug, such as shown as 54a, in a lower chamber 74 is released into the wellbore 12. As object 54a passes second signal system 46, the signal from first signal system 44is received by second signal system 46, and a signal is transmitted tomonitoring system 102, notifying an operator that the operation cancontinue. Upon receipt of the signal triggered by passage of the firstobject 54 a, a cementious slurry is released into the wellbore.Following the release of the cementious slurry, a second plug, such asshown as 54 b, in an upper chamber 76 is released into the wellbore 12.As object 54 b passes second signal system 46, the signal from firstsignal system 44 is received by second signal system 46, and a signal istransmitted to monitoring system 102, notifying an operator that theoperation can continue. Upon receipt of the signal triggered by passageof the second object 54 b, drilling or some other type of working fluidmay be released into the wellbore to complete the operation. It will beappreciated that the operator is relying on the received trigger signal,indicating that the object has moved past the second signal system 46,before proceeding with a particular operation.

While the foregoing has been described with the first signal system 44carried by the object 54 as a transmitter signal and the second signalsystem 46 positioned along the travel path of the object 54 as a signalreceiver, it will be appreciated that the first signal system 44 carriedby object 54 could be a signal receiver and that the second signalsystem 46 positioned along the travel path of the object 54 could be asignal transmitter, so long as the transmitter and receiver operate inconjunction to identify the passage of the object 54 past a knownlocation along the travel path Likewise, it will be appreciated that thetravel path may be above the wellhead 32 to track movement of an object54 through or past surface mounted equipment 62, pressure controldevices 34, subsea conduit 36 or the like; through the wellhead 32; orbelow the wellhead 32 through or past a subsurface equipment 42 or aportion of pipe system 50 disposed within wellbore 12.

Turning to FIG. 4, in one or more embodiments of an object trackingmethod and system for use with oil and gas wellbores 12, the firstsignal system 44 carried by object 54 is disposed to communicate withone or more second signal systems 46 via a through-the-earth transmittedsignal, such as a very-low-frequency (VLF) electromagnetic radiation inthe range of 3-30 kilohertz (kHz) as the signal. The through-the-earthor VLF signal can be used to track an object 54 as it passes through thewellhead 32 and into the portion of pipe system 50 within formation 14via a VLF signal conveyed through formation 14, thereby allowing anoperator to know if an object 54 has by through a location orsub-surface equipment or has reached a particular depth.

In embodiments utilizing a through-the-earth or VLF signal, one or moresurface second signal systems 60 deployed adjacent surface 16 aredisposed to communicate via a through-the-earth or VLF signal. In thisregard, in some embodiments, at least two spaced apart through-the-earthor VLF second signal system 60 are deployed adjacent surface 16 so thata 2-dimensional position of object 54 can be determined bytriangulation, trilateration or similar algorithms or techniquesutilized to determine location. Likewise, through-the-earth or VLFsecond signal system 60 may be arranged on surface 16 in a one or twodimensional array so that a two dimensional or three dimensionalposition of object 54 can be determined by triangulation, trilaterationor similar algorithms or techniques utilized to determine location. Suchalgorithms or techniques can be carried out, for example, by monitoringsystem 102, with data being transmitted to monitoring system 102 via awireless communication path or a wired communication path, which forpurposes of the disclosure may include traditional cable, Ethernet,fiber optics or any other physical transmission medium. VLF secondsignal systems 60 may likewise be in communication with each other via awired or wireless communication path. In some embodiments, three or morespaced apart VLF second signal systems 60 are deployed adjacent surface16 so that a three dimensional position of object 54 can be determinedby triangulation, trilateration or a similar algorithms or techniquesutilized to determine location. In some embodiments, the VLF secondsignal systems 60 may be spaced apart from each other at least 10meters. The position of the object 54 can be overlaid or imposed uponthe known location and orientation of the pipe system 50 to determinethe position of the object 54 in pipe system 50. Alternatively, movementof object 54 can be used to map pipe system 50 as object 54 passestherethrough, thereby permitting the creation of an accuratetwo-dimensional or three-dimensional visualization of wellbore 12 information 14.

In one or more embodiments, the through-the-earth or VLF signal istransmitted at predetermined time intervals, such as every 1-3 seconds,and tracking can occur in real time. Moreover, the first signal system44 is time-synchronized with the second signal system(s) 46 so that thesignal travel time between the first signal system 44 and each secondsignal system 46 can be utilized in the algorithms and techniquesreferenced herein. In this regard, the second signal system(s) 46 may betime-synchronized with each other as well as with the monitoring system102.

It will be appreciated that unlike other EM signals such as radiofrequency (RF) signals that are generally in the frequency range ofapproximately 300 kilohertz (kHz) or higher, through-the-earth signals,which for purposes of this disclosure are below approximately 300 kHzand in particular VLF signals, which are most commonly in the range of3-35 kHz, can penetrate materials such as rock, concrete, metal, andhigh density ore and propagate therethrough. Thus, the VLF signalsystems as described herein may be referred to as through-the-earthsignal systems or VLF signal systems to the extent a communicationssignal is being transmitted through the earth between a transmitter anda receiver.

The disclosure is not limited to a particular arrangement for the firstsignal system 44 and through-the-earth second signal system 60. Whilefirst signal system 44 may be either a transmitter or a receiver orboth, and while through-the-earth or VLF second signal system 60 may beeither a corresponding receiver or a transmitter or both, for purposesof the discussion of FIG. 4, first signal system 44 shall be describedas a VLF or through-the-earth transmitter and through-the-earth or VLFsecond signal system 60 shall be described as a through-the-earth or VLFreceiver.

Thus, through-the-earth or VLF signals will travel through the formation14 from first signal system 44 to through-the-earth or VLF second signalsystem 60. As used herein, a VLF receiver is any receiver disposed toreceive a through-the-earth or VLF signal propagating through a bodysuch as the formation. Such a VLF receiver 60 may include, withoutlimitation, a microphone, a geophone, a single or multi-axisaccelerometer, an acoustic receiver, an optic receiver (such as opticcable) or the like.

Preferably, the VLF receiver 60 is in direct or indirect physicalcontact with the formation so as to form a physical coupling throughwhich a VLF signal may travel. As used herein, a VLF signal may includedata or simply comprise a VLF pulse emitted from first signal system 44acting as a VLF transmitter, thus forming a through-the-earthcommunication system. Although not limited to a particularconfiguration, in one or more embodiments, a through-the-earth signaltransmitter may be a set of electrodes establishing a through-the-earthor VLF electric current or modulated electric carrier waves.

In other embodiments, alternatively, or in addition to through-the-earthor VLF second signal system 60, one or more second signal systems 46 maybe deployed along the wellbore 12 as wellbore second signal systems 58at known spaced apart locations. In one or more embodiments, thesewellbore second signal systems 58 may be disposed to communicate by VLFsignal, RF signal or both, thus permitting movement of object 54 to betracked through pipe system 50.

In one or more embodiments, wellbore second signal systems 58 isdisposed to communicate via through-the-earth or VLF signaltransmissions. In one or more embodiments wellbore second signal systems58 are coupled in direct physical contact with the formation at thewellbore sandface or may be deployed within the cement surroundingwellbore 12 in coupled indirect physical contact with the formation 14so as to form a physical coupling through which a VLF signal may travel.Such wellbore second signal systems 58 may be utilized either to simplyidentify an object 54 as it passes a particular location, much like asdescribed with respect to above-the-wellhead second signal systems 46,or in spaced apart orientation to determine a position or location ofobject 54 as described herein.

In one or more embodiments, one or more of the wellbore second signalsystems 58 may be or include RF sonic or other non-VLF, VLF, EM or othertypes of receivers deployed along pipe system 50. In this regard,wellbore second signal systems 58 may include multiple types of signalreceivers, such as both VLF receivers and another EM receiver, such ornon-VLF or a different frequency VLF receivers. Thus a particular signalmay travel as one type of signal along a first portion of a transmissionpath between the object and a receiver and then along a second portionof the transmission path as a second type of signal. For example, acontrol signal transmitted to the object may travel first as a VLFsignal through the formation to a wellbore second signal system 58,where the signal is converted to a RF signal for line-of-sight or radiofrequency transmission to the object 54 in the wellbore. The signal isconverted from VLF or VLF frequency to RF or RF frequency, or viceversa, for transmission along the transmission path. In another example,a control signal transmitted to the object may travel first as a VLFsignal through the formation to a wellbore second signal system 58,where the signal is converted to an acoustic signal that is transmittedback up the wellbore 12 via a fluid column.

Likewise, object 54 may include at least two transmitters, such as forexample, at least one first transmitter 44 a that may be a VLFtransmitter and at least one second transmitter 44 b that may be an RFtransmitter. Persons of ordinary skill in the art will appreciate thatfirst transmitter 44 a and second transmitter 44 b may be the sametransmitter configured to transmit communication signals at differentfrequencies.

In one more embodiments, as illustrated in FIG. 5, each of surface VLFsecond signal systems 120 transmits a VLF signal into the formation 14.Object 54 receives the transmitted VLF signal from each surface VLFsecond signal systems 120 and then utilizes the received VLF signalsfrom each of the surface VLF second signal systems 120 to determinelocation within wellbore 12. In this regard, location determination canbe performed locally by object 54 or the received VLF signals can becommunicated up wellbore 12 to monitoring system 164 where locationdetermination techniques can be performed. In the case of the latter,information about the received VLF signals can be transmitted up thewellbore utilizing an RF signal generated from object 54 and transmittedup the wellbore 12 by one or more RF wellbore second signal systems 58to monitoring system 102, where the signal data can be utilized todetermine the location of object 54 relative to the surface VLF secondsignal systems 120. Thus, in these embodiments, the signal transmissionpath is down through the formation 14 and then up the wellbore 12 andthe signal travels first as a VLF signal and then as an RF signal, whilein the previous embodiments, the signal traveled as a VLF signal up fromthe object 54 through formation 12 either to surface VLF second signalsystems 120 and/or wellbore second signal systems 58, or both.

To the extent wellbore second signal systems 58 are disposed tocommunicate an RF signal, wellbore second signal systems 58 may bespaced apart along the wellbore so that object 54 is in RF communicationwith at least one RF wellbore second signal system 58 regardless of thelocation of object 54 within pipe system 44. These RF wellbore secondsignal systems 58 may be in communication with each other in order totransmit the signal back up the wellbore 12 to monitoring system 102, orthey may be in some other direct or indirect communications link withmonitoring system 102, such as via a wire.

In one or more embodiments, one or more wellbore second signal system 58include both a VLF transmitter 98 and a RF receiver 100. An RFtransmitter 101 may be carried on object 54 and disposed to transmit anRF signal as the object 54 moves along wellbore 12. As the object 54passes or otherwise is within a predetermined range of a particular RFreceiver 100, the RF receiver triggers its associated VLF transmitter 98to transmit a VLF signal through the earth to the VLF surface secondsignal systems 60 positioned on surface 16.

In one or more embodiments, a two-way communication link can beestablished between the surface second signal systems 60 and firstsignal system 44 on object 54, thus allowing remote activation ofdownhole equipment, such as tools, packers, valves, diverters and thelike. Specifically, in some embodiments, a VLF signal transmitted fromsurface second signal systems 60 through formation 14 may be received byfirst signal system 44. Alternatively, the VLF signal may be received bywellbore second signal systems 58 and utilized to trigger thetransmission of a control signal to object 54 or subsurface equipment42. To the extent the signal is transmitted to object 54, object 54 cansubsequently transmit an RF signal to another wellbore second signalsystem 58 deployed along pipe system 50 to activate subsurface equipment42 or object 54 can communicate directly with the subsurface equipment42, providing the control signal. In these embodiments, a portion of theactivation or control signal is transmitted as a VLF signal through theearth and a portion of the activation or control signal is transmittedas a RF signal.

FIG. 6 illustrates one or more embodiments of an object tracking methodand system for use with wellbore drilling and production system 10 whereat least two, and preferably three or more surface second signal systems60 are deployed adjacent surface 16 at spaced apart positions orlocations 136. The positions or locations 136 are known or determinedutilizing a positioning or location system 138 to determine absolutepositioning of second signal systems 60 and object 54 relative to oneanother. In one embodiment, four through-the-earth surface second signalsystems 60 are deployed adjacent surface 16 at spaced apart positions orlocations 136. Surface second signal systems 60 may be arranged onsurface 16 in a one or two dimensional array. In one or more embodimentswhere at least 3 surface second signal systems 60 are used, the at leastthree receivers are spaced apart from one another so as to form atriangular grid or pattern on the surface 16.

In one or more preferred embodiments, each through-the-earth secondsignal system 60 may include a VLF receiver 99 as described herein. Inany event, such positioning system 138 may include one or more globalpositioning system (GPS) receivers, accelerometers (single ormulti-axis), magnetometers, (single or multi-axis), theodolites,compasses, or, any kind of optical or physical system that can be usedto measure the surface position or location 136 of each second signalsystem 60 on surface 16 and generate surface location data that can beassociated with a through-the-earth tracking signal received from object54 at each particular second signal system 60 second signal system 60.Positioning system 138 may operate utilizing one or more GPS satellites140 such as is illustrated in FIG. 6. In one or more embodiments, fouror more GPS satellites 140 are utilized. In one or more embodiments, thepositioning system 138 is a GPS receiver 142 associated with each secondsignal system 60 second signal system 60 so as to form an overall“underground” GPS to track object 54 in wellbore 12. In other words,positioning system 138, such as a GPS system or other surfacepositioning device, is used to accurately determine the location 136 ofthe point on surface 16 where a particular through-the-earth or VLFsignal is received. The absolute locations of multiple second signalsystem 60 and hence the location 136 where each through-the-earth signalis received or transmitted, combined with timing information related tothe through-the-earth signals between the object 54 and each secondsignal system 60 can be used to determine the three-dimensional positionof the object 54 within the formation 14. Although the disclosure is notlimited to a particular technique, in one or more embodiments, suchposition may be determined by triangulation, trilateration or a similaralgorithm or geometric techniques utilized to determine location. Suchposition determination can be carried out, for example, by monitoringsystem 102, with data being transmitted to monitoring system 102 via awireless communication path or wired communication path, which forpurposes of the disclosure may include traditional cable, Ethernet,fiber optics or any other physical transmission medium. Second signalsystems 60 may likewise be in communication with each other via a wiredor wireless communication path. In one or more embodiments, each secondsignal system 60 includes a dedicated positioning system 138, such asGPS receiver, which GPS receiver 138 may be integrated with secondsignal system 60. In such case, the GPS data can be updated duringtracking operations, thereby enhancing underground tracking results. Thesystem, and in particular, a combined second signal system 60 andpositioning system 138 having a GPS receiver, function as amplifiers, inthat the actual GPS signal is received at the surface and magnified for“underground” use.

In other embodiments, a positioning system 138 having a GPS receiversimply may be utilized to place each second signal system 60 at apredetermined location 136 or to identify the location 136 where asecond signal system 60 is placed, thereby generating absolutepositioning coordinates of each second signal system 60 that cansubsequently be used in location determination of object 54. Whether asecond signal system 60 has a dedicated positioning system 138 having aGPS receiver or a GPS receiver 138 is simply used in the placement ofsecond signal system 60, for purposes of the disclosure, each secondsignal system 60 is said to have a GPS receiver 138 associated with it.In some embodiments, the positioning system 138 having a GPS receivermay be combined with a second signal system 60 to form an integral,standalone second signal system package 144 for deployment along surface16, wherein a plurality of the packages 144 comprise the object trackingsystem. Moreover, the term “receiver” as used herein may includetransmitters or transceivers, such as for example, the referenced GPSreceiver 142 may receive and transmit signals with a GPS satellite as iswell known in the industry.

Although any type of through-the-earth energy signal may be utilized inconjunction with location system 138, in one or more preferredembodiments, the through-the-earth signal may be a VLF signal asdescribed above. In one or more embodiments, the through-the-earthenergy signals may be acoustic or pressure energy. In one or moreembodiments, the through-the-earth energy may be EM energy at otherfrequencies.

An operation 200 to identify the position of an object 54 in formation14 is illustrated in FIG. 7. In a first step 202, multiple second signalsystems 60 are deployed on the surface 16, preferably spaced apart fromone another to form an array, above wellbore 12. The second signalsystems 60 are time synchronized with each other and with the firstsignal system 44 carried by object 54, all of which may also be timesynchronized with a monitoring system 164. In a second step 204, object54 is released into wellbore 12. Prior to release, a first signal system44 may be activated to communicate with one or more second signal system46 via a through-the-earth or VLF signal, such as, for example, secondsurface signal systems 60 positioned on surface 16.

In step 206, the absolute or relative position of each second signalsystems 60 is determined utilizing a GPS system or other locationdetermination system. In one or more preferred embodiments, each secondsignal systems 60 includes a GPS receiver 12 and during a positiondetermination operation, is in continuous or semi-continuouscommunication with a system of GPS satellites. Alternatively, the GPSreceiver 12 of each second signal systems 60 may be intermittentlyactivated to determine location. Time synchronization may occur via theGPS system. In any event, the relative positioning of each second signalsystem 60 is determined.

In step 208, the first signal system 44 and the second surface signalsystems 60 communicate with one another via a through-the-earth signaltransmitted therebetween. In one or more embodiments, thethrough-the-earth signal is emitted into the formation 14 by firstsignal system 44 carried by object 54. Each surface second signal system60 receives the through-the-earth signal. In one or more otherembodiments, each second surface signal system 60 may transmit athrough-the-earth signal that is received by the first signal system 44.In either case, in one or more embodiments, each second surface signalsystem 60 connects to one or more GPS satellites 140 via a GPS receiver142 associated with each second surface signal system 60.

In step 210, the signals received by the systems 60 are utilized totriangulate or otherwise determine the position of the object 54 inwellbore 12 utilizing the three-dimensional positioning (x,y,z) of eachsecond surface signal system 60, the orientation (φ,ψ,θ) of the firstsignal system 44 and the distance (d) between the first signal system 44and each second surface signal system 60. This determination may becarried out at a base station, such as monitoring system 164, that isdirectly or indirectly in communication with each second surface signalsystem 60 or the first signal system 44, or both.

Turning to FIG. 8, in one or more embodiments, first signal system 44carried by object 54 is a piezoelectric system 300 disposed to trigger asignal when pressure is applied to piezoelectric system 300.Piezoelectric system 300 includes one or more piezoelectric elements302. In one or more embodiments, piezoelectric system 300 includes aplurality of piezoelectric elements 302 arranged to about one another toform a stack 304. In any event, piezoelectric element 302 is mounted onobject 54 so as to have at least one exterior surface 306 arranged so asto be exposed to pressure applied from fluid within a wellbore, as isexplained below.

Although piezoelectric element 302 may be any shape without liming thedisclosure, in some embodiments, element 302 may be a disk 308 with anaperture 310 formed through disk 308. As such, when a plurality of disks308 are arranged to form a stack 304, the apertures 310 align to form apassage or through way 312 extending through the stack 304. Disk 308 maybe square, round or have any other perimeter shape as desired. In theembodiments illustrated in FIG. 8, disk 308 is round in shape andaperture 310 is circular in shape, such that through way 312 is a boreextending through the stack 304. The stack 304 may be comprised ofpiezoelectric element 302 each with brass electrodes between eachelement (+and −) to improve structural integrity. Alternatively, stack304 may be a single piezoelectric element 302, with only a single brasselectrode on each end.

In one or more embodiments, one or more first piezoelectric elements 302a may form a first stack 304 a and one or more second piezoelectricelements 302 b may form a second stack 304 b. In such case, each stack304 a, 304 b may be selected to respond to a different stimulus, i.e., adifferent threshold pressures.

In one or more embodiments, a protective coating 314 is applied to orover exterior surfaces 306. Protective coating 314 may be formed of anymaterial that allows a pressure applied thereto to be passed to exteriorsurface 306. Although the disclosure is not limited to a particularprotective coating, in one or more embodiments, protective coating 314may be formed of parylene, silicon, an elastomer or similar materialthat will permit the transmission of force to the exterior surface 306while protecting the piezoelectric elements 302 from the hightemperature, corrosive environment characteristic of wellbores. It willbe appreciated that parylene may be particularly desirable because itcan be applied directly on the surface 306, conforms to the shape ofsurface 306, is effectively stress-free, is chemically and biologicallyinert and stable, and is resistant to solvents and corrosive chemicals,such as may be found in downhole environments.

Although neither object 54 nor piezoelectric system 300 carried byobject 54 are limited to a particular type or configuration, FIG. 8illustrates one embodiment of object 54 and piezoelectric system 300,wherein piezoelectric system 300 forms a cylindrical stack 304. Object54 may include an elongated tubular body 106 having a first end 108 anda second end 110 with a bore 112 formed therein. In one or moreembodiments, bore 112 may be a throughbore to allow fluid to passthrough object 54.

Disposed along the outer surface of object 54 are one or more wipers 114having an outwardly extending flexible lip 116. Lip 116 may be formed ofa pliable or resilient material, such a rubber. An end cap 118 ismounted on first end 108 and may include an aperture 120 incommunication with bore 112. Piezoelectric system 300 comprises firstsignal system 44 and is mounted within bore 112. Through bore 312 ofstack 304 is in fluid communication with bore 112 so that fluid enteringobject 54 through aperture 120 may pass through object 54.

Piezoelectric system 300 may include electronics 316 to convert anelectric charge generated by deformation of a piezoelectric element 302or the stack 304 into a signal that can be transmitted to a secondsignal system 46. In one or more embodiments, electronics 316 may bedisposed to generate and transmit an acoustic signal which can travel upwellbore 12 through a fluid column for receipt by a second signal system46 in the form of a microphone. In this regard, electronics 316 mayinclude a power source 318, such as a battery, to facilitate generationof a signal. Although the disclosure is not limited to a particularoperation of piezoelectric system 300 and electronics 316, in one ormore embodiments, piezoelectric system 300 and electronics 316 may becalibrated to respond once a minimum pressure value (reaction pressure)has been achieved along exterior surface 306. Likewise, due to thenature of piezoelectric element 302, the response signal willincrementally change with pressure. This is also desired as it willprovide a better/more clear indication of when object 54 lands andexperiences a “bump pressure.”

One or both ends of stack 304 may be bounded by a ceramic spacer 305.The hardness of a ceramic spacer allows better energy transfer from themotion of the stack 304 to the fluid medium. In this regard, in someembodiments, stack 304 motion is designed to be axial so any radialcomponent would not be significant.

In one or more embodiments, the signal transmitted by the piezoelectricsystem 300 may be an acoustic wave (0-20 kHz) to communicate datathrough the fluid column to surface in order to determine the landinglocation of object 54.

In any event, piezoelectric system 300 is secured to object 54 by afastener 124, preferably adjacent the second end 110 of object 54 tofacilitate transmission of a signal up wellbore 12. It will beappreciated that in the foregoing arrangement, exterior surface 306 isthe surface of through bore 312 of stack 304 so that pressure may beapplied to stack 304 by wellbore fluid passing through tubular body 106.

In one or more embodiments, fastener 124 may be an externally threadedring disposed to engage internal threads disposed in bore 112 adjacentsecond end 110 of object 54. A protective cover 318 having an aperture320 formed therein may be secured to tubular body 106 to inhibit largerdebris from passing into through bore 312 as object 54 is pumped downinto a wellbore 12. In one or more embodiments, protective cover 318 isformed of an elastomer or other pliant material.

In operation, object 54 is released into a wellbore 12. Although object54 may travel by gravity, in one or more embodiments, it is carried by aservicing fluid pumped down wellbore 12. It will be appreciated that asobject 54 is generally traveling down wellbore 12, the pressure acrossexterior surface 306 is approximately the pressure of the servicingfluid in the wellbore 12. In other words, the pressure at the first end108 and second end 110 are approximately the same as the object travelsuninhibited along a wellbore 12.

In any event, object 54 travels along wellbore 12 until object 54 landson a seat or landing collar 56 (see FIG. 1), which is disposed forreceipt of object 54. Upon landing on seat or collar 56, it will beappreciated that object 54 functions to decrease the cross-sectionalflow path of the servicing fluid, hence increasing the pressure of thefluid column upstream of seat or collar 56. In this regard, the flowpath may be directed primarily through a channel or passageway, such asthroughbore 184 of object 54, along which the exterior surface 306 ispositioned. The increased pressure of the fluid along the flow pathresults in an increase in the pressure applied to exterior surface 306of piezoelectric system 300 as the fluid flows past object 54. Inresponse to the increase in pressure on exterior surface 306, thepiezoelectric elements 302 generate an electrical charge resulting froman applied mechanical force. In one or more embodiments wherepiezoelectric system 300 includes a stack 304 of circular piezoelectricelements 302 forming a through bore 312, once object 54 has landed onseat or collar 56, downward fluid flow is directed though through 312such that through bore 312 functions as a constriction in the flow ofthe service fluid, and thus increasing the pressure of the fluid flowingalong through bore 312. This increased pressure results in an outwardradial force on exterior surface 306, thus resulting in the generationof a charge by piezoelectric elements 302.

The charge generated by the piezoelectric elements 302 can then be usedby the piezoelectric system 300 to produce and transmit a signal to asecond signal system 46, which second single system 46 may be adjacentthe surface 16, or incorporated in a downhole tool or system 42, orotherwise deployed in the wellbore, such as wellbore second signalsystem 58. In particular, electronics 316 receive the generated electriccharge and transmits a signal. The signal may be a an EM signal, an RFsignal, a VLF signal, a through-through-the signal as described above,or any other type of signal. In one or more preferred embodiments, thesignal may be an acoustic signal. In this regard, piezoelectric elements302 may be utilized by piezoelectric system 300 to generate an acousticsignal for transmission up the wellbore 12 through the fluid column,such as by utilizing a power source 318 to drive piezoelectric elements302 at a particular frequency. In this embodiment, the signal may be anacoustic signal that propagates up the fluid column in the wellbore andthe second signal system 46 may be a microphone in communication withmonitoring system 102.

It will further be appreciated that the piezoelectric system 300 isadjustable so that the piezoelectric system 300 will only generate asignal once a particular pressure threshold has been reached. Thisallows an operator to distinguish between a circumstance where theobject may become lodged in the wellbore at a location other than thedesired seat. Thus, for example, in an instance where the object lodgesalong the wellbore at a location other than the desired seat, a pressureincrease may be experienced in the fluid column upstream of the object54, but not a pressure increase to the degree that would trigger asignal from piezoelectric system 300. Alternatively, a pressure increasemay occur that is above a threshold expected when the object 54 seats inthe desired location. In such case, in some embodiments, a signal isgenerated by the piezoelectric system 300 when a lower threshold isreached, and another signal is generated if a second upper threshold isreached. For example, the lower threshold may signify to an operatorthat object 54 has lodged or seated somewhere along the wellbore 12, butan upper threshold may signify that the object 54 is not seated in thedesired location, resulting in a larger pressure than would be expectedif the service fluid were flowing past the object 54 as desired.

Thus, a system for tracking an object in surface mounted equipment of anoil and gas wellbore has been described. Embodiments of the foregoingsystem may generally include a releasable object, the releasable objectincluding a first signal system; and a second signal system positionedin proximity to the surface mounted equipment and disposed tocommunicate with the first signal system Likewise, a surface mountedsystem for an oil and gas wellbore has been described and may generallyinclude a tubular member having a first end and a second end; an objectrelease mechanism in communication with the first end of a tubularmember; a releasable object releasably contained within the releasemechanism, the releasable object including a transmitter; and a receiverpositioned in proximity to the surface mounted equipment and disposed toreceive a signal from the transmitter. Likewise, a system for trackingan object in an oil and gas wellbore within a formation has beendescribed and may generally include a releasable object disposed in awellbore extending from the surface of the formation, the releasableobject including a first VLF signal system; and at least two second VLFsignal systems coupled to the surface and disposed to communicate withthe first signal system via a VLF signal. Relatedly, a releasable objectfor release into an oil and gas wellbore has been described and maygenerally include a body; and a VLF transmitter carried by the body.Similarly, a system for tracking an object in an oil and gas wellborewithin a formation has been described and may generally include areleasable object disposed in a wellbore extending from the surface ofthe formation, the releasable object including a first through-the-earthsignal system; at least three second through-the-earth signal systemscoupled to the surface and disposed to communicate with the first signalsystem via a through-the-earth signal; and a positioning systemassociated with each second signal system. A system for tracking anobject in an oil and gas wellbore within a formation may also generallyinclude a releasable object disposed in a wellbore extending from thesurface of the formation, the releasable object including a first signalsystem, wherein the first signal system comprises and RFID transmitter;at least three second signal systems coupled to the surface, wherein thesecond signal systems are through-the-earth signal systems; a pluralityof third signal systems spaced apart from one another along a wellboreand coupled to the formation and disposed to communicate with the firstsignal system via a through-the-earth signal, each third signal systemincluding an RFID reader; and a positioning system associated with eachsecond signal system. A releasable object for release into an oil andgas wellbore may also generally include a body; and a piezoelectricsystem carried by the body. A system for performing an operation in awellbore may generally include a body; a first signal system carried bythe body, wherein the first signal system comprises a piezoelectricelement; and a second signal system disposed to communicate with thefirst signal system.

For any of the foregoing embodiments, the system or object may includeany one of the following elements, alone or in combination with eachother: a first signal system comprises a transmitter and the secondsignal system comprises a receiver; a first signal system comprises areceiver and the second signal system comprises a transmitter; areceiver further comprises a wireless transmitter in wirelesscommunication with a monitoring system; a releasable object is selectedfrom the group consisting of plugs, balls, and darts; a transmitter isan RFID chip;

a transmitter comprises a magnetic material; a releasable object is aplug comprising: an elongated tubular body having a first end and asecond end with a bore formed therein, wherein the transmitter is acylindrical shaft formed of a signal emitting material and mounted inthe bore; a wiper disposed along an outer surface of the plug, the wiperhaving an outwardly extending flexible lip; a piezoelectric systemcarried by a releasable object; a bore formed in the tubular body is athroughbore, and a cylindrical shaft forming a transmitter includes athroughbore in fluid communication with the throughbore of the tubularbody; a plug further comprises an end cap mounted adjacent the first endof the elongated tubular body, the end cap including an aperture influid communication with the throughbore of the tubular body, whereinthe cylindrical shaft is mounted adjacent the second end of the tubularbody; the body of a releasable object is selected from the groupconsisting of a ball, a plug, or a dart; the body of a releasable objectis a plug comprising: an elongated tubular body having a first end and asecond end with a bore formed therein, wherein the transmitter ismounted in the bore; an RF transmitter carried by the body of areleasable object; the VLF transmitter comprises a piezoelectricelement; a first signal system comprises a VLF transmitter and thesecond signal system comprises a VLF receiver; a first signal systemcomprises a VLF receiver and the second signal system comprises a VLFtransmitter; a transmitter is disposed to transmit a VLF signal in therange of 3-35 kilohertz (kHz); two VLF receivers are spaced apart fromone another on the surface; at least three VLF receivers spaced apartfrom one another on the surface; the VLF receiver is a microphone; a VLFreceiver disposed along the wellbore and coupled to the formation; aplurality of spaced apart VLF receivers disposed along the wellbore andcoupled to the formation; an RF receiver or RF transmitter incommunication with at least one VLF receiver; a plurality of spacedapart RF receivers disposed along the wellbore, and wherein thereleasable object further includes a RF transmitter; a transmitter isdisposed to emit a unidirectional signal; shielding disposed to limitthe direction of propagation of a signal from the transmitter; atransmitter comprises a material that emits an electromagnetic signal; areceiver is an RFID reader; a receiver is an electromagnetic sensor; areceiver is a sensor; an object release mechanism in communication withthe first end of a tubular member, wherein a receiver is positionedalong the tubular member between the first end and a second end of thetubular member; a surface mounted system is a cement head assembly antthe tubular member forms an elongated tubular therein, the cement headassembly further comprising an inner bore formed in the elongatedtubular and extending therethrough, wherein the object release mechanismcomprises a first object chamber and a second object chamber formed in aportion of the inner bore, a release mechanism disposed in proximity toeach of first chamber and second chamber, each release mechanismincluding a release element movable between a first position to secure areleasable object in an associated chamber and a second position torelease a releasable object from the associated chamber, wherein thereceiver is positioned along the elongated tubular between the secondobject chamber and the second end of the tubular member; a receiverfurther comprises a wireless transmitter in wireless communication witha monitoring system; a transmitter is selected from the group consistingof an RFID chip, a magnetic material and a material that emits anelectromagnetic signal, and wherein the receiver is selected from thegroup consisting of an RFID reader and sensor; a cement head assemblyfurther comprises an upper safety valve system and a lower safety valvesystem; a movable release element of cement head assembly is a rotatablecylindrical element having a first radial through bore and rotatablebetween the first position in which release element bore is offset fromelongated tubular inner bore and the second position in which therelease element bore is substantially aligned with the elongated tubularinner bore; a release mechanism of cement head assembly furthercomprises a position indicator externally mounted along the elongatedtubular; a flipper mechanism of cement head assembly is positioned alongelongated tubular between the second chamber and the second end of thetubular member, said flipper mechanism comprising an extension movablymounted on elongated tubular so that the extension protrudes into theinner bore when the extension is in a first position and is at leastpartially retracted from the inner bore when the extension is in asecond position, a visual indicator mounted on the exterior of theelongated tubular and lined to the extension; the positioning system isselected from the group consisting of GPS receivers, accelerometers(single or multi-axis), magnetometers, (single or multi-axis),theodolites, compasses and optical systems; the second signal systemsare through-the-earth transmitters and the third signal systems arethrough-the-earth receivers; the second signal systems arethrough-the-earth receivers and the third signal systems arethrough-the-earth transmitters; the positioning system is a globalpositioning system (GPS) comprising a GPS receiver; the positioningsystem is selected from the group consisting of GPS receivers,accelerometers (single or multi-axis), magnetometers, (single ormulti-axis), theodolites, compasses and optical systems; the firstsignal system is a transmitter and the second signal systems arereceivers; the first signal system is a receiver and the second signalsystems are transmitters; the second through-the-earth signal systemsare spaced apart from one another so as to form a triangular grid; eachsecond through-the-earth signal system further comprises a separate GPSreceiver with each second through-the-earth signal system; the secondthrough-the-earth signal systems are spaced apart from one another onthe surface a distance of at least 10 meters; one of either the first orsecond through-the-earth signal system is disposed to transmit a VLFsignal in the range of 3-30 kilohertz (kHz) and each of the otherthrough-the-earth signal system is disposed to receive the VLF signal; athrough-the-earth signal transmitter is a set of electrodes establishingthe VLF electric current or modulated electric carrier waves; thethrough-the-earth receiver is selected from the group a microphone, ageophone, a single or multi-axis accelerometer, an acoustic receiver oran optic receiver; at least four of the second through-the-earth signalsystems spaced apart from one another on the surface; a monitoringsystem disposed to receive data from each of the secondthrough-the-earth signal systems; a wireless communication networkbetween each of the second through-the-earth signal systems and amonitoring system; the releasable object is a plug comprising: anelongated tubular body having a first end and a second end with a boreformed therein, wherein the first through-the-earth signal system is atransmitter mounted in the bore; a portion of the through-the-earthsignal systems is selected from the group consisting of a microphone, ageophone, a single or multi-axis accelerometer and an acoustic receiver;the piezoelectric system comprises a piezoelectric element; thepiezoelectric system comprises a plurality of piezoelectric elements inabutting contact with one another to form a piezoelectric stack; theplurality of piezoelectric elements each comprises a disk with anaperture formed therein and the apertures form a throughbore extendingthrough the stack; the piezoelectric stack defines an exterior surface,the piezoelectric system further comprising a coating disposed over theexterior surface; the coating is select from the group consisting ofparylene, silicon, and an elastomer; the body further comprises a fluidflow passage formed therein, and at least a portion of the piezoelectricsystem is disposed along the flow passage; the body is a plug comprisingan elongated tubular body having a first end and a second end with abore formed therein, wherein the a piezoelectric system comprises acylindrical stack of piezoelectric elements mounted in the bore; thebore formed in the tubular body is a throughbore, and the cylindricalstack includes a throughbore in fluid communication with the throughboreof the tubular body; the plug further comprises an end cap mountedadjacent the first end of the elongated tubular body, the end capincluding an aperture in fluid communication with the throughbore of thetubular body, wherein the cylindrical stack is mounted adjacent thesecond end of the tubular body; the releasable object further comprisesa protective cover with an aperture formed therein, the cover disposedadjacent the second end of the tubular body; the releasable objectfurther comprises an EM transmitter; the piezoelectric system comprisesa first set of piezoelectric elements and a second set of piezoelectricelements carried by the body, wherein the first set of piezoelectricelements is disposed to generate a first signal in response to a firstpressure and the second set of piezoelectric elements is disposed togenerate a second signal in response to a second pressure different fromthe first pressure; the first signal system comprises a plurality ofpiezoelectric elements and the second signal system is a microphone; thefirst signal system further comprises control electronics electricallyattached to the piezoelectric element and a power source; the bodyfurther comprises a fluid flow passage formed therein, and at least aportion of the piezoelectric element is disposed along the flow passage.

Thus, a method for tracking an object released adjacent surface mountedequipment of an oil and gas wellbore has been described and maygenerally include positioning a receiver between the surface mountedequipment and the wellhead of a wellbore; transmitting a signal from areleasable object; releasing the object to pass through at least aportion of the surface mounted equipment; and utilizing the receiver toverify that the releasable object has passed through at least a portionof surface mounted equipment. A method for tracking the position anobject released into a wellbore has been described and may generallyinclude positioning a first VLF signal system along the surface of aformation in which the wellbore is formed; releasing a releasable objectinto a wellbore; transmitting a VLF signal through the earth between thereleasable object and the first VLF signal system; and determining theposition of the object in the wellbore based on the transmitted VLFsignal. A method for tracking the position an object released into awellbore also may generally include releasing a releasable object intothe wellbore; and determining the position of the object in the wellboreutilizing a global positioning system. A method for performing anoperation in a wellbore has been described and may generally includereleasing an object into a wellbore; pumping a service fluid in thewellbore to urge the object along the wellbore; utilizing apiezoelectric element carried by the object to generate a signal whenthe object engages a seat.

For the foregoing embodiments, the method may include any one of thefollowing steps, alone or in combination with each other: verifyingcomprises transmitting an RFID signal from the releasable object andidentifying the RFID signal as the releasable object passes in proximityto the receiver; verifying comprises transmitting a magnetic signal fromthe releasable object and identifying the magnetic signal as thereleasable object passes in proximity to the receiver; transmitting asignal comprises activating a transmitter carried by the releasableobject; transmitting a signal to a control system removed from thesurface mounted equipment;

releasing at least two objects to pass through at least a portion of thesurface mounted equipment; and utilizing the receiver to verify thateach released object has passed through at least a portion of surfacemounted equipment, wherein each released object emits a differentsignal; operating an object release system to release a first plug froma cement head assembly into a wellbore; verifying that the first plughas passed in proximity to the receiver; wireles sly transmitting afirst signal to a monitoring system; upon receipt of the wirelesslytransmitted first signal, releasing a cementious material into thewellbore behind the first plug and thereafter, operating an objectrelease system to release a second plug from the cement head assemblyinto the wellbore; verifying that the second plug has passed inproximity to the receiver; wirelessly transmitting a second signal to amonitoring system; and upon receipt of the wirelessly transmitted secondsignal, releasing a working fluid into the wellbore behind the secondplug; transmitting a first signal by the first plug and transmitting asecond signal by the second plug that is different than the firstsignal; positioning at least two first VLF signal systems along thesurface of a formation, the first VLF signal systems being spaced apartfrom one another on the surface; transmitting a VLF signal through theearth between the releasable object and each first VLF signal system;and utilizing triangulation among the first VLF signal systems and theobject to determining the position of the object in the wellbore;positioning three or more first VLF signal systems along the surface ofa formation and utilizing VLF signals transmitted between each first VLFsignal system and the object to determine the position of the object inthe wellbore by triangulation; measuring the travel time of thetransmitted VLF signal between each first VLF signal system and theobject; measuring the distance between each first VLF signal system; andutilizing the measured travel times and distances to determine theposition of the object in the wellbore;

spacing the first VLF signal systems apart at least 10 meters from oneanother along the surface; determining the location of the objectrelative to the first VLF signal systems and overlying a threedimensional grid of the wellbore with the determined position of theobject relative to the first VLF signal system; transmitting the VLFsignal through the formation from the object to the first VLF signalsystem; transmitting the VLF signal through the formation from first VLFsignal system to the object; generating a VLF signal in the range of3-30 kilohertz (kHz); transmitting the VLF signal at predetermined timeintervals; coupling the first VLF signal system in physical contact withthe formation so as to form a physical coupling through which a VLFsignal may travel; deploying along the wellbore in a known location atleast one first VLF signal system; and utilizing the first VLF signalsystem deployed in the wellbore to track movement of the object alongthe wellbore; coupling the first wellbore VLF signal system in physicalcontact with the formation so as to form a physical coupling throughwhich a VLF signal may travel; mounting a first wellbore VLF signalsystem receiver in casing cement; mounting a first wellbore VLF signalsystem in contact with the wellbore sandface; coupling one or more firstVLF signal systems to one or more RF receivers and transmitting a signalbetween the surface and the object first as a VLF signal and then as andRF signal; electrically coupling one or more first VLF signal systems toone or more RF receivers and transmitting a signal between the surfaceand the object first as a RF signal and then as and VLF signal;utilizing a global positioning system in determining the position of theobject in the wellbore; utilizing a global positioning system indetermining the position of the first VLF signal system; utilizing athrough-the-earth transmission signal to communicate between at leastthree through-the-earth signal systems disposed adjacent the surface ofa formation and the object;

transmitting a through-the-earth signal from the object to each of thesurface signal systems and utilizing the signal received by each surfacesignal system in determining the position of the object in the wellbore;transmitting a through-the-earth signal from each of the surface signalsystems to the object and utilizing each of the signals received by theobject in determining the position of the object in the wellbore;utilizing the global positioning system to ascertain the location of thereceivers on the surface; generating positional GPS data for each of thereceivers on the surface and transmitting the GPS data to a controlsystem; time synchronizing a through-the-earth signal using the globalpositioning system; the generated signal is an electrical chargegenerated by the piezoelectric element; the generated signal is from adeformation of the piezoelectric element; directing a fluid flow througha passage in the releasable object to increase the pressure of the fluidand utilizing the increased pressure applied to the piezoelectricelement to generate an electrical charge; transmitting an acousticsignal through a fluid column in response to the generated signal;transmitting a signal from the object in response to the generatedsignal; the transmitted signal is selected from the group consisting ofan EM signal, an RF signal, a VLF signal, a through-the-earth signal oran acoustic signal; the transmitted signal is generated utilizing thepiezoelectric element; and a first signal is transmitted uponapplication of a first pressure to a piezoelectric element and a secondsignal upon application of a second pressure to a piezoelectric element.

While the foregoing disclosure is directed to the specific embodimentsof the disclosure, various modifications will be apparent to thoseskilled in the art. It is intended that all variations within the scopeand spirit of the appended claims be embraced by the foregoingdisclosure

What is claimed:
 1. A releasable object for release into an oil and gaswellbore, the releasable object comprising: a body; and a piezoelectricsystem carried by the body.
 2. The releasable object of claim 1, whereinthe body is selected from the group consisting of a ball, a plug, or adart.
 3. The releasable object of claim 1, wherein the piezoelectricsystem comprises a piezoelectric element.
 4. The releasable object ofclaim 3, wherein the piezoelectric system comprises a plurality ofpiezoelectric elements in abutting contact with one another to form apiezoelectric stack.
 5. The releasable object of claim 4, wherein theplurality of piezoelectric elements each comprises a disk with anaperture formed therein and the apertures form a throughbore extendingthrough the stack.
 6. The releasable object of claim 4, wherein thepiezoelectric stack defines an exterior surface, the piezoelectricsystem further comprising a coating disposed over the exterior surface.7. The releasable object of claim 6, wherein the coating is select fromthe group consisting of parylene, silicon, and an elastomer.
 8. Thereleasable object of claim 1, wherein the body further comprises a fluidflow passage formed therein, and at least a portion of the piezoelectricsystem is disposed along the flow passage.
 9. The releasable object ofclaim 1, wherein the body is a plug comprising an elongated tubular bodyhaving a first end and a second end with a bore formed therein, whereinthe a piezoelectric system comprises a cylindrical stack ofpiezoelectric elements mounted in the bore.
 10. The releasable object ofclaim 9, wherein the bore formed in the tubular body is a throughbore,and the cylindrical stack includes a throughbore in fluid communicationwith the throughbore of the tubular body.
 11. The releasable object ofclaim 1, further comprising an EM transmitter.
 12. The releasable objectof claim 1, wherein the piezoelectric system comprises a first set ofpiezoelectric elements and a second set of piezoelectric elementscarried by the body, wherein the first set of piezoelectric elements isdisposed to generate a first signal in response to a first pressure andthe second set of piezoelectric elements is disposed to generate asecond signal in response to a second pressure different from the firstpressure.
 13. A method for performing an operation in a wellbore, themethod comprising: releasing an object into a wellbore; pumping aservice fluid in the wellbore to urge the object along the wellbore; andutilizing a piezoelectric element carried by the object to generate asignal when the object engages a seat.
 14. The method of claim 13,wherein the generated signal is from a deformation of the piezoelectricelement.
 15. The method of claim 13, further comprising, directing afluid flow through a passage in the releasable object to increase thepressure of the fluid and utilizing the increased pressure applied tothe piezoelectric element to generate an electrical charge.
 16. Themethod of claim 13, further comprising transmitting an acoustic signalthrough a fluid column in response to the generated signal.
 17. Themethod of claim 13, further comprising transmitting a signal from theobject in response to the generated signal.
 18. The method of claim 17,wherein the transmitted signal is selected from the group consisting ofan EM signal, an RF signal, a VLF signal, a through-the-earth signal oran acoustic signal.
 19. The method of claim 17, further comprisinggenerating the transmitted signal utilizing the piezoelectric element.20. The method of claim 17, wherein a first signal is transmitted uponapplication of a first pressure to a piezoelectric element and a secondsignal upon application of a second pressure to a piezoelectric element.21. A system for performing an operation in a wellbore, the systemcomprising: a body; a first signal system carried by the body, whereinthe first signal system comprises a piezoelectric element; and a secondsignal system disposed to communicate with the first signal system. 22.The system of claim 21, wherein the first signal system comprises aplurality of piezoelectric elements and the second signal system is amicrophone.
 23. The system of claim 21, wherein the body furthercomprises a fluid flow passage formed therein, and at least a portion ofthe piezoelectric element is disposed along the flow passage.